We are able to observe nearly any kind of astronomical objects in the universe. But one thing, we will never be able to observe: the black dwarf! The black dwarf is a hypothetic kind of star which has developed from a white dwarf. Sometime in the far future, every white dwarf will lapse into a black, cold object without any light emission and a temperature of nearly 0 K, the so-called black dwarf. We can logically conclude that this will happen sometime, but we will never see or observe such an object due to the massive amount of time that must go by until it will happen. Astronomers say that a white dwarf will become a black dwarf after at least 1 quadrillion (10^15) years. This is a time span by far longer than the age of the present universe. The universe is not old enough to contain black dwarves.

And it also could be, that the universe will never contain black dwarves in the future. If the universe will end up in a big rip or a big crunch, it lasts not long enough. Only if the universe will end up in a big chill, black dwarves can be really possible. That's weird. I don't know if you ever heard about that black dwarf hypothesis.

Another event we will never observe is the end of the universe itself. There are three different hypotheses (big crunch, big chill and big rip). How do you think the universe will end? Which of the three hypotheses do you espouse?